1. Field of the Invention
The invention relates generally to induced polarization surveying and, more particularly, but not by way of limitation, it relates to a system for deriving both time domain and frequency domain indications from spatially related induced polarization data.
2. Description of the Prior Art
The prior art includes many variations of the two basic types of induced polarization system, i.e. a time domain system or a frequency domain system. In either type of system current is applied to the earth through a pair of current electrodes, and resulting voltage differences are measured at a spaced distance across a pair of potential electrodes which are also disposed in conductive contact with the earth's surface. Data or indications determinative of different depths of penetration may be adjusted by the off-set or spacing between the current electrodes and the potential electrodes. Previously, the time domain types of system have applied a periodically interrupted d-c current to the earth with subsequent measurement of the earth voltage decay versus time after removal of current application. An integration of the decay curve provides a relative measure of the polarizability of the earth; however, such data is seriously affected by presence of noise.
Prior art frequency domain techniques have been utilized where a fixed period square wave current is applied to the earth through current electrodes, and the resulting voltage difference is measured at spaced receivers or potential electrodes. Thus, data is then repeated for a square wave current of same amplitude but different period to generate data for evaluation in the frequency domain. A difference between voltages measured for the two successive input frequencies is a relative measure of the polarizability of the earth at that location, but once again the presence of noise contributes to great deterioration of result.